The mGluR5 receptor is a Type I metabotropic glutamatergic receptor which is critically involved in a number of brain disorders including psychostimulant drug abuse, the Fragile X syndrome, Huntington's disease, depression, and anxiety disorders as well as playing an important role in motor dyskinesias in Parkinsons disease. Despite its importance in these disorders there has been a paucity of data in humans regarding mGluR5 function due to a lack of appropriate imaging methods. Recently new PET radioligands for the mGluR5 receptors have been discovered; animal and initial human studies suggest that [18F]FPEB is the most promising. Before [18F]FPEB can be used in clinical research, studies in humans of radiation dosimetry of [18F]FPEB , validation of methods of quantitation of regional mGluR5 levels, and the test-retest reliability of these estimates need to be performed. The R21 phase of this application will provide these data which are needed for design of future clinical research PET studies of the mGluR5 receptor. The R33 phase of this application will examine the role of the mGluR5 receptor in methamphetamine (METH) abuse. While the development of METH addiction has been related to its ability to markedly and rapidly elevate extracellular dopamine levels, animal studies indicate that repeated psychostimulant administration produces increased mGluR5 receptor levels and a high tonic level of mGluR5 mediated neurotransmission in the ventral striatum which appears to be a critical factor in the maintenance of METH addiction. Selective mGluR5 antagonists reverse both the increased tonic mGluR5 signaling and the addictive effects of METH in animals. Other studies indicate that decreased frontal cortical mGluR5 receptor levels are seen in depression and that frontal cortical mGluR5 neurotransmission mediates cognitive functions including executive function which are impaired in METH abusers. Depressive symptoms and impaired executive function in METH abusers are significant risk factors for drug relapse. The goals of the R33 phase of this application are to measure regional mGluR5 levels in METH abusers using PET [18F]FPEB studies, and to correlate regional mGluR5 receptor levels with the altered reward behaviors, cognitive impairments, psychiatric symptoms seen in METH abusers and with lifetime METH use. The proposed studies will be the first studies the of mGluR5 receptor in METH abuse in humans, and will significantly enhance our knowledge of the role of the mGluR5 receptor in humans abusing METH.